Upstream eventBinding of antagonist, NMDA receptors
Key Event Relationship Overview
AOPs Referencing Relationship
|AOP Name||Adjacency||Weight of Evidence||Quantitative Understanding|
|Chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development induces impairment of learning and memory abilities||adjacent||High|
|Chronic binding of antagonist to N-methyl-D-aspartate receptors (NMDARs) during brain development leads to neurodegeneration with impairment in learning and memory in aging||adjacent||High|
Life Stage Applicability
Key Event Relationship Description
It is well documented that prolonged/chronic antagonism of NMDARs triggers the downstream KE named inhibition of NMDARs. Shorter term binding to the same receptors may trigger different downstream KEs, such as up-regulation of the NMDARs, resulting in toxic increased influx of calcium and to cell death. Consequently, this information can be captured in other KERs and AOP.
Evidence Supporting this KER
There is structural mechanistic understanding supporting the relationship between MIE (NMDARs, binding of antagonists) and KE (NMDARs, inhibition). Crystal structure studies are used to study the binding of antagonists/agonists to NMDA receptors. In case of NMDAR antagonists, the binding to the receptor causes LBD conformation changes which promote channel closure leading to reduced Ca+2 influx (Blanke and VanDongen, 2009). This lack of measurable ion flux is applied as an indication of NMDAR inhibition.
Include consideration of temporal concordance here
In slices of cerebellum derived from postnatal days 6-30 (PND 6-30) Sprague Dawley rats, 10 µM MK-801 completely blocked evoked NMDA excitatory postsynaptic currents (EPSCs) as it has been demonstrated by patch clamp technique (Rumbaugh and Vicini, 1999). The same technique has been employed in cortical slices from C57BL/6 mice of both genders and different age groups (P8-12, P21-28 or P45-90), showing that 1 µM APV and 50 nM NVP-AAM077 antagonise a similar amount of NMDA receptor current independently of the age (de Marchena et al., 2008).
Pb acts as a non-competitive, voltage independent antagonist of NMDAR. Pb action partially overlaps with that of zinc (Gavazzo et al., 2008)
Pb2+ has potent inhibitory effects on the NMDA receptor (Alkondon et al., 1990; Guilarte and Miceli, 1992; Guilarte, 1997; Gavazzo et al., 2001). In rat hippocampal neurons, Pb2+ (2.5-50 μM) inhibits NMDA-induced whole-cell and single-channel currents in a concentration-dependent manner, suggesting that Pb2+ can decrease the frequency of NMDA-induced channel activation (Alkondon et al., 1990). In the same study, they have examined the effect of Pb2+ on the binding of [3H]MK-801 to the rat brain hippocampal membranes and showed that Pb2+ inhibits the binding of [3H]-MK-801 in a concentration dependent manner with an IC50 value close to 7 µM (Alkondon et al., 1990). These inhibitory effects of Pb2+ on NMDA receptors activation appear to be age and brain region specific (Guilarte, 1997; Guilarte and Miceli, 1992). The Pb2+ IC50 is significantly lower in cortical membranes prepared from neonatal than from adult rats, whereas the hippocampus is more sensitive than the cerebral cortex since the Pb2+ IC50 is significantly lower in the hippocampus (Guilarte and Miceli, 1992). The number of [3H]-MK-801 binding sites associated with the high and low affinity sites of Pb2+ inhibition in the hippocampus of rats is increased as a function of age, peaking at PND 28 and 21 (Guilarte, 1997). High and low affinity Pb(2+)-sensitive [3H]-MK-801 binding sites have also been measured in the cerebral cortex during early development, but that has not been possible to evaluate after PND 14.
The developing brain is more sensitive than the adult brain to Pb2+-induced effects mediated through the NMDA receptor. Moreover, the hippocampus appears to be particularly vulnerable as in this brain structure NMDA receptors undergo subunit specific changes during developmental Pb2+ exposure (Guilarte and McGlothan, 1998). Exposure to Pb2+ during synaptogenesis causes decreased expression of hippocampal NR2A-subunit of NMDARs at synapses and increased targeting of NR2B-NMDARs to dendritic spines (without increased NR2B-NMDARs expression) (Nihei and Guilarte, 1999; Neal et al., 2011; Zhang et al., 2002).
Uncertainties and Inconsistencies
Pb2+ has been found to produce either potentiation or inhibition depending on: a) the subunit composition of NMDA receptors, b) endogenous glutamate concentration and c) Pb2+ dosage. In case that the NMDA receptors are saturated by agonist, Pb2+ at low concentrations (<1 µM) acts as a positive modulator of agonist action at NR1b-2AC and NR1a-2AB subunit complexes, whereas at higher concentrations, Pb2+ it behaves as a potent inhibitor of all recombinant NMDA receptors tested and was least potent at NR1b-2AC (Omelchenko et al., 1996; 1997), meaning that Pb2+ is not always acting as NMDAR inhibitor but it can also behave as NMDAR activator under certain conditions.
As an alternative mechanism of toxicity, Pb was shown to cause oxidative stress. In addition, it has the ability to substitute other bivalent cations like Ca2+,Mg2+, Fe2+ and monovalent cations like Na+ (for review, see Flora et al., 2012)
Quantitative Understanding of the Linkage
Is it known how much change in the first event is needed to impact the second? Are there known modulators of the response-response relationships? Are there models or extrapolation approaches that help describe those relationships?
To predict how potent an antagonist can be, the half maximal inhibition concentrations (IC50) and the half maximal effective concentration (EC50) of glutamate/glycine induced currents is measured in NMDA receptors from brain slices and cells or in recombinantly expressed receptors. Traynelis et al. 2010 summarised the IC50 values for competitive, noncompetitive and uncompetitive antagonists in different subunits of NMDA receptors. The inhibitory effect (efficacy) of antagonists on NMDA receptors has been found to be dependent on:
-the type of subunits that form the NMDA receptors depending on the developmental stage -the chemical structure of the antagonists
-the binding site of receptor that the antagonists prefer
-how tightly an antagonist binds to the receptor (affinity)
At CA3-CA1 synapses, NMDARs are largely composed of NR1 (NMDA receptor subunit 1)-NR2A or NR1-NR2B containing subunits. Recent, but controversial, evidence has correlated NR1-NR2A receptors with the induction of LTP and NR1-NR2B receptors with LTD. However, LTP can be induced by activation of either subtype of NMDAR and the ratio of NR2A:NR2B receptors has been proposed as an alternative determinant of the direction of synaptic plasticity (Mac Donald et al., 2006).
Pb2+: Although the NR2 subunits have different Zn2+ binding sites i.e. the NR2A-NMDAR binds Zn2+ at a high-affinity site (nM affinity) while the NR2B-NMDAR binds Zn2+ with lower affinity (µM range); the Pb2+ IC50 for wild type NR2A-NMDARs was reported to be 1.3 µM, while the Pb2+ IC50 of wild type NR2B-NMDARs was 1.2 µM (Gavazzo et al., 2008). Similar findings were published by Lasley and Gilbert (1999) using cortical neurons from adult rats. The IC50 for Pb2+ ranged from 1.52 to 4.86 µM, with the ranking of Pb2+ potency in inhibition of NMDA receptor subunits to be NR1b-2A>NR1b-2C>NR1b-2D>NR1b-2AC after experiments that have been conducted in Xenopus oocytes injected with cRNAs for different combinations of NMDA receptor subunits (Omelchenko et al., 1997).
Known modulating factors
Known Feedforward/Feedback loops influencing this KER
Domain of Applicability
The biophysical properties of rat and human receptors have been mostly assessed through recombinant studies, whereas the pharmacological properties of rat and human NMDA receptors have not been fully explored and compared yet (Hedegaard et al., 2012). Mean channel open times for human NMDA receptor subtypes in recombinant protein studies are similar to those of the corresponding rat NMDA receptor subtypes. However, mean single-channel conductances for human NMDA receptor subtypes appear lower than those of the corresponding rat NMDA receptor subtypes. Regarding pharmacological properties of the receptors, the differences were less than 2-fold and were not observed at the same subtypes for all the antagonists tested, suggesting that the molecular pharmacology of NMDA receptor is conserved between human and rat, although some inter-species differences are seen in IC50 values using two-electrode voltage-clamp recordings (Hedegaard et al., 2012),
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Blanke ML, VanDongen AMJ. (2009) Activation Mechanisms of the NMDA Receptor. In: Van Dongen AM, editor. Biology of the NMDA Receptor. Boca Raton (FL): CRC Press; Chapter 13. Available from: http://www.ncbi.nlm.nih.gov/books/NBK5274/
de Marchena J, Roberts AC, Middlebrooks PG, Valakh V, Yashiro K, Wilfley LR, Philpot BD. (2008) NMDA receptor antagonists reveal age-dependent differences in the properties of visual cortical plasticity. J Neurophysiol. 100: 1936-1948.
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Omelchenko IA, Nelson CS, Allen CN. (1997) Lead inhibition of N-Methyl-D-aspartate receptors containing NR2A, NR2C and NR2D subunits. J Pharmacol Exp Ther. 282: 1458-1464.
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